The ecumenical councils, at least the first 6, are a prerequisite for understanding what Christ actually did. We can know they’re true because the alternatives all present internal contradictions.

For example, if the Nestorians are right, the atonement doesn’t make any sense. Anyone could’ve been sacrificed and it wouldn’t have made a difference.

If the monophysites are correct, the nature of God changed into a created substance. I don’t even need to go into all the problems that creates with Biblical claims about God.

The ecumenical councils, from the Lutheran perspective at least, aren’t true because the consensus has divine protection. They’re true because their proclamations are just self-evidently true and consistent with Scripture.